1. Field of the Invention
The present invention relates to a control system for controlling a belt-type continuously-variable transmission, and more particularly to a control system for controlling the speed ratio of a belt-type continuously variable transmission which is hydraulically operated to control the speed ratio through a four-way valve.
2. Description of the Prior Art
Belt-type continuously variable transmissions have drive and driven pulleys each having a variable pulley width and a metallic V-belt trained around the drive and driven pulleys. Such belt-type continuously variable transmissions are finding increasing use as motor vehicle transmissions. To control the speed ratio of the belt-type continuously variable transmissions, the hydraulic pressure of working oil supplied to a hydraulic cylinder for setting the pulley width of the drive pulley and also the hydraulic pressure of I working oil supplied to a hydraulic cylinder for setting the pulley width of the driven pulley, i.e., the axial thrust force of the pulleys, are controlled to vary the pulley widths of the pulleys.
Heretofore, it has been customary to supply the hydraulic cylinder combined with the driven pulley with a hydraulic pressure that is required to keep the belt under tension and also to supply the hydraulic cylinder combined with the drive pulley with working oil which has such a hydraulic pressure through a flow control valve for controlling the speed ratio. The pulley width of the drive pulley is controlled by the supplied working oil. Since the hydraulic pressure varies depending on the speed ratio and the input torque of the transmission, the rate at which the speed ratio is controlled (speed ratio control rate) is affected by the speed ratio and the input torque of the transmission.
It has been known to employ a four-way valve to supply working oil to one of cylinders combined with respective drive and driven pulleys and discharge working oil from the other cylinder for controlling the speed ratio of a belt-type continuously variable transmission. For example, Japanese laid-open patent publication No. 62-196447 discloses a transmission control apparatus for controlling the speed ratio of a belt-type continuously variable transmission. In the disclosed transmission control apparatus, a first line pressure regulated by a first pressure regulating valve and a second line pressure, lower than the first line pressure, regulated by a second pressure regulating valve are supplied selectively to the cylinders combined with the respective drive and driven pulleys through the four-way valve. Due to the difference between the first and second line pressures, one of the cylinders is supplied with the working oil, whereas the other cylinder discharges the working oil.
According to the disclosed control process, a control value is determined to obtain a hydraulic pressure Pin' in the cylinder combined with the drive pulley in order to generate an axial thrust force for achieving a target speed ratio, and the first line pressure is determined by adding a corrective hydraulic pressure .DELTA.P1 to the hydraulic pressure Pin'. If the corrective hydraulic pressure .DELTA.P1 is reduced, then the constant state deviation is increased, resulting in a larger difference between the present speed ratio and the target speed ratio, so that the speed ratio cannot be controlled accurately. Conversely, if the corrective hydraulic pressure .DELTA.P1 is increased, then the steady state deviation is reduced (though it cannot be reduced to zero), but the first line pressure may unnecessarily become high, resulting in a large power loss.
Another problem is that inasmuch as the corrective hydraulic pressure .DELTA.P1 affects the speed ratio control rate, if the corrective hydraulic pressure .DELTA.P1 is reduced, then the speed ratio control rate is lowered, and hence the response of the speed ratio control is also lowered. Conversely, if the corrective hydraulic pressure .DELTA.P1 is reduced, then the speed ratio control rate is increased. However, the controlled speed ratio tends to overshoot the target speed-ratio, and the speed ratio control suffers hunting.
Proportional plus integral (PI) control or proportional pulse integral plus derivative (PID) control processes may be introduced to prevent the overshooting and eliminate the constant state deviation. However, such PI or PID control processes take a relatively long period of time until the speed ratio reaches the target speed ratio, and hence lowers the speed ratio control response.